Wind turbine bat mortality is a significant adverse impact of large scale wind energy development. Wind energy has become an increasingly important sector of the renewable energy industry, and may help to satisfy a growing worldwide demand for electricity. Environmental benefits of wind energy accrue from the replacement of energy generated by other means (e.g., fossil fuels, nuclear energy), reducing some adverse environmental effects from those industries. However, development of the wind energy industry has led to some unexpected environmental costs. For example, soaring and feeding raptors have been killed in relatively large numbers in areas of high raptor abundance in the United States and Europe. More recently, large numbers of bat fatalities have been observed at utility-scale wind energy facilities, especially along forested ridgetops in the eastern US. Similar fatalities have been reported at wind energy facilities in Europe. As such facilities continue to develop in other parts of the world, especially in Australia, China, and India. increased numbers of bat and bird fatalities can be expected.

The degree of bat mortality impact is thought to be understated in the literature, not only due to difficulties of carcass retrieval, but also due to inadequate number of surveys; the implication of the latter is that catastrophic events of large scale bat kill may not be documented since some areas of the world involve concerted bat movements of such intensity that a single night of migration intercepted by a wind turbine array could decimate a local population Extreme examples of such movements are seen at the forested ridge above Deer Cave in Gunung Mulu National Park in Borneo, although there is presently no wind energy project in this area.

Utility-scale wind energy development in the US

In 2005, utility-scale wind energy facilities in the US accounted for approximately 9616 megawatts (MW) of installed capacity. The number and size of wind energy facilities have continued to increase, with taller and larger turbines being constructed. Available estimates of installed capacity in the US by 2020 range up to 72,000 MW, or the equivalent 48 000 1.5 MW wind turbines. This is enough, according to some projections, to account for 5% of the country’s electrical generating capacity. Most existing wind energy facilities in the US include turbines with installed capacity ranging from 600 kW to 2 MW per turbine. Wind turbines up to about 3 MW of installed capacity for onshore applications are currently being tested. However, owing to seasonally variable wind speeds, the generating capacity of most existing wind turbines is less than 30% of installed capacity.

Utility-scale wind turbines (> 1 MW) installed in, or planned for, the US since the 1990s are designed with a single monopole (tubular tower), ranging in height from 45 to 100 meters (m), with rotor blades up to 50 m in length. At their greatest height, blade tips of typical 1.5 MW turbines may extend to 137 m (as tall as a 40-story building with a rotor diameter the size of a 747 jumbo jet). Typically, wind turbines are arranged in one or more arrays, linked by underground cables that provide energy to a local power grid (WebFigure 1). Some modern turbines (eg GAMESA G87 2.0 MW turbine) rotate up to 19 rpm, driving blade tips at 86 m s–1 (193 mph) or more. Since utility-scale wind turbines were first deployed in the US in the 1980s, the height and rotor-swept area has steadily increased with each new generation of turbines.

To date, most utility-scale wind turbines in the US have been installed in grassland, agricultural, and desert landscapes in western and mid-western regions. More recently, however, wind turbines have been installed along forested ridgetops in eastern states (Figure 1). More are proposed in this and other regions, including the Gulf Coast and along coastal areas of the Great Lakes. Large wind energy facilities off the coastline of the northeastern US have also been proposed.

Bat fatalities

Relatively small numbers of bat fatalities were reported at wind energy facilities in the US before 2001, largely because most monitoring studies were designed to assess bird fatalities. Thus, it is quite likely that bat fatalities were underestimated in previous research. Recent monitoring studies indicate that some utility-scale wind energy facilities have killed large numbers of bats. Of the 45 species of bats found in North America, 11 have been identified in ground searches at wind energy facilities (Table 1). Of these, nearly 75% were foliage-roosting, eastern red bats (Lasiurus borealis), hoary bats (Lasiurus cinereus), and tree cavity-dwelling silver-haired bats (Lasionycteris noctivagans), each of which migrate long distances (Figure 2). Other bat species killed by wind turbines in the US include the western red bat (Lasiurus blossivilli), Seminole bat (Lasiurus seminolus), eastern pipistrelle (Perimyotis [=Pipistrellus] subflavus), little brown myotis (Myotis lucifugus), northern long-eared myotis (Myotis septentrionalis), long-eared myotis (Myotis evotis), big brown bat (Eptesicus fuscus), and Brazilian free-tailed bat (Tadarida brasiliensis). A consistent theme in most of the monitoring studies conducted to date has been the predominance of migratory, tree-roosting species among the fatalities.

For several reasons (e.g., cryptic coloration, small body size, steep topography, overgrown vegetation), bats may have been overlooked during previous carcass searches. Based on recent evaluations of searcher efficiency, on average, only about half of test subjects (fresh and frozen bats or birds) are recovered by human observers.

To date, no fatalities of state or federally listed bat species have been reported; however, the large number of fatalities of other North American species has raised concerns among scientists and the general public about the environmental friendliness of utility-scale wind energy facilities. For example, the number of bats killed in the eastern US at wind energy facilities installed along forested ridgetops has ranged from 15.3 to 41.1 bats per MW of installed capacity per year. Bat fatalities reported from other regions of the western and mid-western US have been lower, ranging from 0.8 to 8.6 bats MW–1yr–1, although many of these studies were designed only to assess bird fatalities.

Table 1. Species composition1 of annual bat fatalities reported for wind energy facilities in the United States, modified from Johnson (2005)

Species2

Pacific
Northwest

Rocky
Mountains

South-
Central

Upper
Midwest

East

Total

Hoary bat

153 (49.8%)

155 (89.1%)

10 (9.0%)

309 (59.1%)

396 (28.9%)

1,023 (41.1%)

Eastern red bat

–

–

3 (2.7%)

106 (20.3%)

471 (34.4%)

580 (23.3%)

Western red bat

4 (1.3%)

–

–

–

–

4 (0.2%)

Seminole bat

–

–

–

–

1 (0.1%)

1 (0.1%)

Silver-haired bat

94 (30.6%)

7 (4.1%)

1 (0.9%)

35 (6.7%)

72 (5.2%)

209 (8.4%)

Eastern pipistrelle

–

–

1 (0.9%)

7 (1.3%)

253 (18.5%)

261 (10.5%)

Little brown myotis

2 (0.7%)

6 (3.5%)

–

17 (3.3%)

120 (8.7%)

145 (5.8%)

Northern long-eared myotis

–

–

–

–

8 (0.6%)

8 (0.4%)

Big brown bat

2 (0.7%)

2 (1.1%)

1 (0.9%)

19 (3.6%)

35 (2.5%)

59 (2.4%)

Brazilian free-tailed bat

48 (15.6%)

–

95 (85.5%)

–

–

143 (5.7%)

Unknown

4 (1.3%)

4 (2.2%)

–

30 (5.7)

15 (1.1%)

53 (2.1%)

Total

307

174

111

523

1,371

2,486

1Pacific Northwest data are from one wind energy facility in CA, three in eastern OR, and one in WA; Rocky Mountain data are from one facility in WY and one in CO; Upper Midwest data are from one facility in MN, one in WI, and one in IA; South-Central data are from one facility in OK; East data are from one facility in PA, one in WV, and one in TN.

2One confirmed anecdotal observation of a western long-eared myotis (Myotis evotis) has been reported in CA, but is not included in this table.

Locations of bat fatalities

Bat fatalities at wind energy facilities appear to be highest along forested ridgetops in the eastern US and lowest in relatively open landscapes in the mid-western and western states, although relatively large numbers of fatalities have been reported in agricultural regions from northern Iowa and southwestern Alberta, Canada . To date, no assessments of bat fatalities have been reported at wind energy facilities in the southwestern US, a region where large numbers of migratory Brazilian free-tailed bats are resident during the warm months, and where this species provides important ecosystems services to agriculture.

Seasonal timing of bat fatalities

Most bat fatalities in North America have been reported in late summer and early autumn, and similar seasonal trends have been reported for bats in northern Europe. Migration of tree bats in North America is known to occur from March through May and again from August through November. The few bat fatalities reported during spring migration and early summer may reflect the fact that less intensive fatality searches were conducted during this period, but it may also be due to bats migrating at higher altitudes during spring. Many, if not most, of the bat species that have been killed by wind turbines in the US (Table 1) are resident during summer months. In addition to being at risk during migration, the large colonies of Brazilian free-tailed bats that disperse nightly across vast landscapes in the southwestern US may be at risk during the period of summer residency.

How and why are bats being killed?

It is clear that bats are being struck and killed by the turning rotor blades of wind turbines. It is unclear, however, why wind turbines are killing bats, although existing studies offer some clues. Several mechanisms have been postulated:

Are bats in some way attracted to wind turbines? Some migratory species are known to seek the nearest available trees as daylight approaches, and thus could mistake large monopoles for roost tree. Tree-roosting bats, in particular, often seek refuge in tall trees.

Are bats attracted to sites that provide rich foraging habitats? Modifications of landscapes during installation of wind energy facilities, including the construction of roads and power-line corridors, and removal of trees to create clearings (usually 0.5–2.0 hectares [ha]) around each turbine site may create favorable conditions for the aerial insects upon which most insectivorous bats feed.

Are bats attracted to the sounds produced by wind turbines? Some bat species are known to orient toward distant audible sounds, so it is possible that they are attracted to the swishing sounds produced by the rotating blades. Alternatively, bats may become acoustically disoriented upon encountering these structures during migration or feeding. Bats may also be attracted to the ultrasonic noise produced by turbines.

What other factors might contribute to bat fatalities? Wind turbines are also known to produce complex electromagnetic fields in the vicinity of nacelles. Given that some bats have receptors that are sensitive to magnetic fields interference with perception in these receptors may increase the risk of being killed by rotating turbine blades. Bats flying in the vicinity of turbines may also become trapped in blade-tip vortices (Figure 4) and experience rapid decompression due to changes in atmospheric pressure as the turbine blades rotate downward.

Do some weather conditions place bats at increased risk of being killed by wind turbines? Preliminary observations suggest an association between bat fatalities and thermal inversions following storm fronts or during low cloud cover that force the animals to fly at low altitudes.

Are bats at risk because they are unable to acoustically detect the moving rotor blades? Current evidence is inconclusive as to whether bats [echolocation|echolocate]] during migration, independent of time spent searching for and capturing insects.

Projected cumulative fatalities

We have projected cumulative fatalities of bats at wind energy facilities for the Mid-Atlantic Highlands using data on current fatality rates (Table 1) and projections of installed capacity for wind energy facilities in the Highlands for the year 2020. Projections of installed capacity range from 2158 MW to 3856 MW. Although the estimated number of bat fatalities reported for each study were not consistently corrected for search efficiency or for potential bias associated with carcass removal by scavengers, we have nonetheless used these estimates to project cumulative impacts on bats because they are the only fatality rates available for bats in this region. In making our projections of cumulative fatalities, we have assumed that: (1) current variation in fatality rates is representative of the Mid-Atlantic Highlands, (2) future changes in design or placement of turbines (e.g., more and larger installed turbines) will not cause deviations from current fatality estimates, (3) abundance of affected bat species will not decrease due to turbine-related fatalities or other factors (e.g., habitat loss), and (4) projections of cumulative fatalities for other geographic regions differ from those in the Mid-Atlantic Highlands. The projected number of annual fatalities in the year 2020 (rounded to the nearest 500) range from 33 000 to 62,000 individuals, based on the NREL’s WinDS Model, and 58,997 (59,000) to 110,667 (111,000) bats based on the PJM grid operator interconnection queue. For the three migratory, tree-roosting species from the mid-Atlantic Highlands, the projected cumulative fatalities in the year 2020 based on the WinDS model and PJM grid operator queue, respectively, would include 9500 to 32,000 hoary bats, 11,500 to 38,000 eastern red bats, and 1500 to 6000 silver-haired bats. Given the uncertainty in estimated installed wind turbine capacity for the Mid-Atlantic Highlands and existing data on bat fatalities reported for this region, the above projections of cumulative fatalities should be considered provisional and thus viewed as hypotheses to be tested as improved estimates (or enumerations) of installed capacity and additional data on bat life histories and fatalities become available for this region. Nonetheless, these provisional projections suggest substantial fatality rates in the future. At this time, we have avoided making projections of cumulative fatalities for the entire period from 2006–2020, because of substantial uncertainty with respect to population sizes and the demographics of bat species being killed in this region.

If these and other species-specific projections are realized for the Mid-Atlantic Highlands, there may be a substantial impact on both migratory and local bat populations. Migratory tree-roosting species are of particular concern because these bats have experienced the highest fatality rates at wind energy facilities in North America.

It is unlikely that the eastern red bat (Lasiurus borealis) could sustain cumulative fatality rates associated with wind energy development as projected, given that this species already appears to be in decline throughout much of its range. Major gaps exist in knowledge about the timing, magnitude, and patterns of bat migration and the underlying evolutionary forces that have shaped this seasonal behavior. When lack of knowledge is combined with the fact that bats generally have low reproductive rates, significant cumulative impacts of wind energy development on bat populations can be expected.

Much of the existing data on bat fatalities at wind energy facilities are based on monitoring studies designed primarily for the detection and estimation of bird fatalities. Results from these studies vary considerably with respect to geographic location, landscape conditions, search frequency, season of monitoring, and potential biases for searcher efficiency and carcass removal by scavengers, and search intervals have ranged from 1 to 28 days (Table 2). Because some studies have shown that bats can be scavenged within hours of being killed, there is considerable uncertainty in reported fatality estimates when search intervals longer than a day are used. Moreover, because only six monitoring studies have routinely used bat carcasses to correct for observer bias, the number of reported fatalities provides, at best, a minimum estimate (Table 2).

Research needs

The unexpectedly large number of migratory tree bats being killed by wind turbines and the projected cumulative fatalities in the Mid-Atlantic Highlands should be a wake-up call for those who promote wind energy as being “green” or environmentally friendly. Uncertainties with respect to the projected fatalities as noted above invite comprehensive multi-year surveys and hypothesis-based research to advance our understanding of where, when, how, and why bats are killed at wind energy facilities. Research is needed to develop solutions at existing facilities and to aid in assessing risk at proposed facilities, particularly in landscapes where high bat fatalities have been reported and in regions where little is known about migratory and foraging habits of bats. To advance knowledge about the causes of bat fatalities at wind energy facilities and to help guide the establishment of mitigating solutions, we propose the following research directions:

Conduct full-season (April–November in the continental US, for example), multi-year pre- and post-construction monitoring studies to assess species composition, species abundance, local population variability, and temporal and spatial patterns of bat activity at facilities that encompass diverse landscapes.

Conduct local-, regional-, and continental-scale population estimates of North American bat species. In particular, use of molecular methods to estimate effective population size of species most at risk should be a high priority.

Quantify geographic patterns of bat activity and migration with respect to topography and land cover.

Quantify relationships between bat abundance and fatality risks and the relationship of fatalities to bat demography at local, regional, and continental scales.

Conduct quantitative studies of bat activity at existing wind energy facilities to evaluate how variations in weather and operating conditions of turbines affect bat activity and fatalities. Variables to be evaluated should include air temperature, wind speed and direction, cloud cover, moon phase, barometric pressure, precipitation, and turbine operating status such as rotation rate and cut-in speeds.

Quantify effects of wind turbine design on bat fatalities with respect to height and rotor diameter, base and tip height of rotor-swept areas, distance between adjacent turbine rotor swept areas, and the scale (size) of wind power facilities.

Quantify effects of feathered (i.e., turbine blades pitched parallel to the wind, making them essentially stationary) versus not feathered (i.e., turbine blades pitched angularly to the wind, causing rotation) turbines at different wind speeds and at multiple sites, especially during high-risk, migratory periods.

Develop predictive and risk assessment models, with appropriate confidence intervals, on local, regional, and continental scales to evaluate impacts of wind energy development on bat populations.

Evaluate possible deterrents under controlled conditions and under different operating conditions and turbine characteristics at multiple sites.

Cooperation and research support from the wind industry

As part of the permitting process, owners and developers should be required to provide full access to proposed and existing wind energy facilities and to fund research and monitoring studies by qualified researchers. Research and monitoring protocols should be designed and conducted to ensure unbiased data collection and should be held to the highest peer-review and legal standards.

Summary

To date, bat fatalities reported in the USA have been the highest at wind energy facilities along forested ridgetops in the East. Reported bat fatalities in this region have ranged from 15.3 to 32 bats/MW/y of installed capacity in the Mid-Atlantic Highlands and 31.5 to 41.1 bats/MW/y in Tennessee. By contrast, published reports of fatalities from western and mid-western states range from 0.8 to 8.6 bats/MW/y of installed capacity. While the lowest fatality rates have been observed in western states, few of these studies were designed to monitor bat fatalities, and thus may represent substantial underestimates. The highest fatality rate for bats was reported for the Buffalo Mountain Wind Energy Center, TN (41.6 bat fatalities/MW/y), where estimates were consistently corrected for both search efficiency and scavenging. A recent study conducted at wind energy facilities in an agricultural region in southwestern Alberta, Canada, unexpectedly found fatality rates comparable to those observed in some forested ridgetops in the eastern US. Given that previous monitoring studies in western agricultural and grassland regions reported relatively low fatality rates of bats, high fatality rates in regions with similar landscapes should receive increased attention. High fatality rates can also be expected at wind energy facilities located in the southwestern US, where, to date, no monitoring studies have been conducted.

Future research should focus efforts on regions and at sites where information suggest the greatest potential for adverse affects. Improved documentation, with emphasis on evaluation of causes and cumulative impacts, should be a high priority. There is an urgent need to estimate population sizes of bat species most at risk, especially migrating, tree-roosting species. Moreover, additional data are needed for assessing fatalities caused by other human activities (e.g., agricultural pesticides, heavy metals released from the burning of fossil fuels and other industrial processes, collisions with communication towers) to better place impacts of wind energy development on bats into a broader context. However, these latter studies will be challenging and expensive and should not take priority over research to find solutions for fatalities caused by wind turbines. An important challenge to policy and decision makers is to ensure that owners and developers of wind energy and other energy-generating facilities be required, as part of the permitting process, to fund qualified research designed assess impacts of these facilities on bats and other wildlife.

Results of scientifically sound research and monitoring studies are needed to inform policy and decision makers during the siting, permitting, and operation of renewable energy sources. Although bat fatalities at wind turbines have been reported at nearly every wind energy facility where post-construction surveys have been conducted, few of these studies were adequately designed for estimating bat fatalities and only a few of these studies included a full season or more of monitoring. Rigorous protocols should include reliable estimates of searcher efficiency and scavenger removal to correct fatality estimates for potential biases.

We have identified several important questions, research needs, and hypotheses that will require the cooperation of scientists, the wind energy industry, regulatory agencies, and other stakeholders. In order to avoid or minimize environmental impacts of wind energy development, it is imperative that we learn where, when, how, and why bats are being killed at these facilities. Improved methods are needed to assess both current and cumulative impacts of wind energy facilities on bats at local, regional, and continental scales.

Future developments of wind energy facilities, and expected impacts on bats, depend upon complex interactions of economic factors, technological development, regulatory changes, political forces, and other factors that cannot be easily or accurately predicted at this time. Our preliminary projections of cumulative fatalities of bats for the mid-Atlantic Highlands are likely to be unrealistically low, especially as larger and increasing numbers of wind turbines are installed. Reliable data on bat fatalities and estimates of demographic and effective population sizes for species at risk are needed from all regions of North America to fully understand continental-scale impacts of wind energy development. Until such time, current and projected cumulative fatalities should provide an important wake-up call to developers and decision makers. Additional monitoring and hypothesis-based research is needed to address a growing concern of national and international importance.

Acknowledgments

We thank D Boone and R Webb for providing references on assessing installed capacity of wind energy facilities in the mid-Atlantic Highlands, and D Boone, C Cleveland, P Cryan, and A Manville for their suggestions on earlier versions of this manuscript. This paper evolved as an outgrowth of several state, regional, and national, wind-energy, and wildlife workshops, including:

Bats and Wind Power Generation Technical Workshop (Juno Beach, FL; 19–20 February 2004; sponsored by the US Fish and Wildlife Service [USFWS], Bat Conservation International, National Renewable Energy Laboratory, and American Wind Energy Association)

Wind Power and Wildlife in Colorado (Fort Collins, CO; 23–25 January 2006; sponsored by the Colorado Department of Natural Resources)

Toward Wildlife Friendly Wind Power: A Focus on the Great Lakes (Toledo, OH; 27–29 June 2006; sponsored by US Environmental Protection Agency, USFWS Great Lakes Basin Ecosystem Team, Illinois Natural History Survey, and US Geological Survey)

New York Wind/Wildlife Technical Workshop (Albany, NY; 2–3 August 2006; sponsored by the New York State Energy Research and Development Authority, and New York Department of Environmental Conservation)

Number of birds (*) + number of bats used in bias correction trials / mean number of days that carcasses lasted during trials. Bird carcasses were sometimes used as surrogates of bats in search efficiency trails.

For this facility, the proportion of the 8 trial bats not scavenged after seven days was used to adjust fatality estimates

Two searches (one in late May and one in late June) conducted at each turbine in 2004, and four searches every 14 days conducted at each turbine between 15 May and 15 July in 2005

Authors used a hypothetical range of carcass removal rates derived from other studies (0-79%) to adjust fatality estimates

Number of birds used during six trials; the mean number of days that carcasses lasted was not available; on average 88% of bird carcasses remained two days after placement

Six-week study period from 1 August to 13 September 2004

Weighted mean number of bat fatalities/MW with weights equal to the proportion of 0.66 MW (n = 3 of 18) and 1.8 MW (n = 15 of 18) turbines

WebTable 2. Projected annual numbers of bats fatalities from wind turbines in 2020 in the Mid-Atlantic Highlands based on, based on projections of installed capacity for this region and estimated fatality rates from the eastern U.S. (Table 1). Numbers in parentheses are projected fatalities rounded to the nearest 500.

Estimated installed capacity of 3856 MW based on PJM (electricity grid operator interconnection queue) for the Mid-Atlantic Highlands for the year 2020.

Eastern red bats, hoary bats, and silver-haired bats are the only species in the eastern US known to undertake long-distance migrations (Barbour and Davis 1969).

Estimated species-specific fatality rates are based on data collected in the eastern US (Table 1).

Minimum projected number of fatalities in 2020 is based on the product of 15.3 bat fatalities/MW/y reported from the Meyersdale Wind Energy Center, PA (Table 2) and the projected installed capacity (2158 MW) = 33,017. The species-specific annual minimum number of projected bat fatalities is the product of the species-specific fatality rates (Column 2) and the minimum total number of fatalities (eg for the hoary bat, 0.289*33,017 = 9542).

Maximum projected number of fatalities in 2020 is based on the product of 28.7 bat fatalities/MW/y (average for 2003 and 2004) reported from the Mountaineer Wind Energy Center, WV (Table 2) and the projected installed capacity (2,158 MW) = 61,935. The species-specific annual maximum number of projected bat fatalities is the product of the species-specific fatality rates (Column 2) and the total maximum number of fatalities.

Minimum projected number of fatalities in 2020 is based on the product of 15.3 bat fatalities/MW/y reported from the Meyersdale Wind Energy Center, PA (Table 2) and the projected installed capacity (3856 MW) = 58,997. The species-specific annual minimum number of projected bat fatalities is the product of the species-specific fatality rates (Column 2) and the total minimum projected number of fatalities.

Maximum projected number of bat fatalities in 2020 is based on the product of 28.7 bat fatalities/MW/y (average for 2003 and 2004) reported from the Mountaineer Wind Energy Center, WV (Table 2) and the projected installed capacity (3,856 MW) = 110,667. The species-specific annual maximum number of projected bat fatalities is the product of the species-specific fatality rates (Column 2) and the total maximum projected number of fatalities.

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